MULTICATION DOPED ALPHA-BETA SIALON CERAMICS

Description

TECHNICAL FIELD

[0001] This invention is about alpha and beta SiAlON composite comprising an alpha SiAlON, a beta SiAlON and an intergranular amorphous and/or crystalline phase. Said alpha SiAlON phase was prepared from a multi-cationic mixture including element of calcium, at least one of yttrium and/or a rare earth element with atomic number greater than 62 and at least one of a rare earth element with atomic number equal or smaller than 62.

BACKGROUND OF THE INVENTION

[0002] Silicon nitride and SiAION ceramics are engineering ceramic materials which are characterised by an excellent combination of mechanical properties of stiffness, strength, hardness and toughness which can, in theory, be retained to very high (> 1000. degree. C) temperatures.

[0003] The SiAlONs are based on compositions containing the elements Si, Al, O, N, hence the acronym. The most successful commercial SiAlON (beta SiAlON) has the beta-Si.sub.3 N.sub.4 crystal structure, but with some of the silicon atoms replaced by aluminium atoms and the same number of nitrogen atoms replaced by oxygen atoms to form Si.sub.6-z Al.sub.z O.sub.z N.sub.8-z where 0<z<4.2. The other common SiAlON phase is alpha SiAlON, which has the general composition M.sub.x Si.sub.12-m-n Al.sub.m+n O.sub.n N.sub.16-n, where m represents the number of Si-N bonds replaced by Al-N per unit cell, n represents the number of Si-N bonds replaced by Al-O per unit cell, 0<x<2, and M is one of the cations including Li, Mg, Ca, Y and rare earths (excluding La, Ce).

[0004] Beta SiAlON is a strong engineering ceramic with good oxidation and creep resistance up to 1300.degree.C. Alpha SiAION has excellent hardness, but slightly worse strength, toughness and oxidation resistance than beta SiAlON. By selecting a particular phase, it is possible to define quite precisely an optimum combination of mechanical properties. Combinations of alpha-beta SiAlONs are in thermodynamic equilibrium and so optimised composite materials can be produced in this way.

[0005] SiAlONs are usually formed by mixing Si.sub.3 N.sub.4, Al.sub.2 O.sub.3, AlN powders with one or more metal oxides (often including Y.sub.2 O.sub.3), compacting the powder to the desired shape, and then firing the component at 1750.degree.C for a few hours. The function of the metal oxide is to react with the silica, always present on the surface of each silicon nitride particle, to form a liquid phase, which assists densification. After sintering, the liquid phase, which also contains nitrogen, cools to form an amorphous phase between the SiAlON grains. In subsequent use of these materials, the amorphous phase starts to soften at temperatures slightly above its glass transition temperature (T.sub.g) and the mechanical properties deteriorate rapidly. Even with the most refractory oxide additives, T.sub.g is barely in excess of 1000.degree.C.

[0006] In an attempt to provide a ceramic SiAlON composition, which is usable in high temperature applications, prior art methods and compositions have taught the combination of alpha SiAlON, beta SiAION and intergranular phases.

[0007] US patent 4,563,433 and US patent 4,711,644 disclose a ceramic containing alpha SiAION, beta SiAlON and an intergranular phase. This alpha SiAlON phase is formed by using yttrium and/or other rare earth elements.

[0008] US patent 5,200,374 discloses a ceramic containing alpha SiAION, beta SiAlON and intergranular phase. This alpha SiAION phase is formed by using rare earth elements selected from the group consisting of Ho, Er, Tm, Yb and Lu.

[0009] US patent 5,227,346 and US patent 5 413 972 disclose a ceramic containing alpha SiAlON, beta SiAlON and intergranular phase. This SiAlON material is formed by using a compound selected from the group consisting of oxides and nitrides of Sr, at least one of Ca, Mg, Li or Na and at least one of yttrium or rare earth elements.

[0010] The mentioned prior arts use yttrium and/or rare earth cations except US patent 5,227,346 and US patent 5 413 972. Although using yttrium and/or rare earth cations gives required multiphase SiAlON ceramic materials, microstructure of these ceramics and to greater extent mechanical and/or thermal properties may not be the desired ones. This can be explained by transformation of alpha SiAlON to beta SiAlON in use where there is a depletion of alpha SiAlON phase.

[0011] Primary objective of the present art is to produce a multi-phase SiAlON material with improved properties and stable microstructure at high temperatures. It is also a further objective to design a composition in which the amount of glassy phase can be minimised by using suitable combination of cations.

SUMMARY OF THE INVENTION

[0012] The objective of the present invention is to produce SiAION material containing three phases, especially for use as cutting tool materials. These phases are comprised of alpha and beta SiAlONs and amorphous and/or crystalline grain boundary phase(s). The alpha SiAlON contains element of calcium, at least one of yttrium and/or a rare earth element with atomic number greater than 62 and at least one of a rare earth element with atomic number equal or smaller than 62. The alpha SiAION exists as either equiaxed or elongated grain morphology while beta SiAlON phase exists only in elongated form. By adjusting the relative amounts of these phases, materials can be tailored to give high hardness, strength and toughness both at room and high temperature.

[0013] Present invention discloses incorporation of three different types of cations into alpha SiAlON structure. Calcium, which has the largest alpha SiAlON former, stabilises alpha SiAlON, which does not transform to beta SiAlON after sintering during cooling period and also reduces the amount of residual grain boundary phase. Yttrium or a rare earth element with atomic number greater than 62 also stabilises the alpha SiAlON and increases the hardness of the final material. A rare earth element with atomic number equal or smaller than 62 helps develop elongated grain morphology for both beta and alpha SiAlONs. Combination of three different types of cations significantly reduces the amorphous and/or crystalline grain boundary phase(s) after sintering.

[0014] Further disclosed is a method for producing the multi-phase SiAION ceramic material. The method includes the steps of (a) preparing a mixture of Si.sub.3 N.sub.4, AlN, Al.sub.2 O.sub.3 and a multi-cation mixture composition which includes a compound selected from the group consisting of oxides and nitrides of calcium; at least one compound selected from the group consisting of oxides and nitrides of yttrium and/or a rare earth element with atomic number greater than 62; and at least one compound selected from the group consisting of oxides and nitrides of a rare earth element with atomic number equal or smaller than 62, (b) attrition milling them in water, (c) drying the mixture and (d) pressing at 150 MPa and (e) sintering between 1600-1850.degree.C at least 18 minutes in a gas pressure sintering furnace at a pressure between 1-100 bar.

DETAILED DESCRIPTION OF THE INVENTION

[0015] This invention concerns a multi-phase SiAION ceramic material comprising a mixture of three phases. These phases are comprised of alpha and beta SiAlONs and amorphous and/or crystalline grain boundary phase(s). The alpha. SiAION has a composition of M.sub.x Si.sub.12-m-n Al.sub.m+n O.sub.n N.sub.16-n, where m represents the number of Si-N bonds replaced by Al-N per unit cell, n represents the number of Si-N bonds replaced by Al-O per unit cell, 0<x<2, and M is (i) calcium, (ii) at least one of yttrium and/or a rare earth element with atomic number greater than 62 and (iii) at least one of a rare earth element with atomic number equal or smaller than 62. This phase of alpha SiAlON was found to be a crystalline phase existing as fine grains with equiaxed or elongated grain morphology depending on starting composition. Micrographs were taken by scanning electron microscopy (SEM) and chemical analyses of alpha-SiAlONs were carried out by energy dispersive X-ray analysis (EDX). EDX analysis results showed three different types of cation which include (i) calcium, (ii) at least one of yttrium and/or a rare earth element with atomic number greater than 62 and (iii) at least one of a rare earth element with atomic number equal or smaller than 62.

[0016] The second phase of the SiAlON ceramic material is beta SiAION having the general formula Si.sub.6-z Al.sub.z O.sub.z N.sub.8-z where 0<z<4.2 but in this invention z value is between 0 and 1.6. Micrograph taken by SEM reveals that this phase has elongated shape. The weight percent ratio of alpha to beta SiAION is preferably changing from about 20:80 to about 80:20.

[0017] The third phase of the multi-phase SiAlON ceramic material comprises an intergranular amorphous and/or crystalline phase(s) containing Si, Al, O, N in combination with (i) calcium (ii) at least one of yttrium and/or a rare earth element with atomic number greater than 62 and (iii) at least one of a rare earth element with atomic number equal or smaller than 62. The intergranular amorphous and/or crystalline phase(s) was found to be present in an amount between 0-16 percent by volume based upon total material volume.

[0018] The method of producing the multi phase SiAION material includes the steps of (a) preparing a mixture of Si.sub.3 N.sub.4, AlN, Al.sub.2 O.sub.3 and a multi-cation mixture composition which includes a compound selected from the group consisting of oxides and nitrides of calcium; at least one compound selected from the group consisting of oxides and nitrides of yttrium and/or a rare earth element with atomic number greater than 62; and at least one compound selected from the group consisting of oxides and nitrides of a rare earth element with atomic number equal or smaller than 62, (b) attrition milling them in water, (c) drying the mixture and (d) pressing at 150 MPa and (e) sintering between 1600-1850.degree.C at least 18 minutes in a gas pressure sintering furnace at a pressure between 1-100 bar.

[0019] The following examples serve to illustrate the novel multi-phase SiAlON material of this invention and the method of preparing the novel SiAlON material. The examples are not intended to limit the scope of this invention but only given to help disclose the invention. All percentages are by weight unless otherwise noted.

EXAMPLE 1 (embodiment not falling under the scope of claims 7-9)

[0020] 

Compound	Weight percent
Si.sub.3 N.sub.4	89.34
AlN	5.330
Y.sub.2 O.sub.3	4.800
Sm.sub.2 O.sub.3	0.412
CaCO.sub.3	0.118

[0021] A mixture of five components given above, in the amounts indicated, was mixed by attrition milling with Si.sub.3 N.sub.4 media in water for two hours to form a powder mixture and then appropriate binders, lubricants and plasticisers were added and dried by a spray drier. Dried powders were sieved and pressed by uniaxially with a pressure of 150 MPa. Green pellets were sintered with five step sintering cycle to 1800.degree.C for 2 hours in a gas pressure sintering furnace at a pressure up to 22 bar. The material was fully dense with a density of 3.26 g/cc measured by water immersion. X-ray diffraction showed both alpha and beta SiAlONs. An alpha to beta ratio of 50:50 was obtained.

EXAMPLE 2 (embodiment not falling under the scope of claims 7-9)

[0022] 

Compound	Weight percent
Si.sub.3 N.sub.4	89.22
AlN	5.32
Y.sub.2 O.sub.3	3.99
Sm.sub.2 O.sub.3	1.234
CaCO.sub.3	0.236

[0023] A mixture of five components given above, in the amounts indicated, was mixed by attrition milling and sintered by gas pressure sintering as in Example 1. The material density was measured at 3.25 g/cc. X-ray diffraction showed both alpha and beta SiAlONs. An alpha to beta ratio of 50:50 was obtained.

EXAMPLE 3

[0024] 

Compound	Weight percent
Si.sub.3 N.sub.4	72.52
AlN	13.61
Al.sub.2 O.sub.3	6.61
Yb.sub.2 O.sub.3	2.74
Sm.sub.2 O.sub.3	2.43
CaCO.sub.3	2.09

[0025] A mixture of six components given above, in the amounts indicated, was mixed by attrition milling and sintered by gas pressure sintering as in Example 1. The material density was measured at 3.25 g/cc. X-ray diffraction showed both alpha and beta SiAlONs. An alpha to beta ratio of 75:25 was obtained. The amount of grain boundary phase is significantly reduced in this example.

Claims

1. A multi-phase SiAlON ceramic material having at least a first, second and third phase, comprising:

(a) a first phase of alpha SiAION having the general formula

        M.sub.x Si.sub.12-m-n Al.sub.m+n O.sub.n N.sub.16-n

wherein 0<x≤2 and M is a multi-cationic mixture including

(i) element of calcium;

(ii) at least one of yttrium and/or a rare earth element with atomic number greater than 62;

(iii) at least one of a rare earth element with atomic number equal or smaller than 62.

(b) a second phase of beta SiAlON having the general formula

        Si.sub.6-z Al.sub.z O.sub.z N.sub.8-z

wherein 0<z<4.2, the value of z is 0<z<1.6

(c) a third phase of an intergranular amorphous and/or crystalline nature containing in addition to elements of Si, Al, O and N,

(i) element of calcium;

(ii) at least one of yttrium and/or a rare earth element with atomic number greater than 62;

(iii) at least one of rare earth element with atomic number equal or smaller than 62.

2. The ceramic material of claim 1, wherein the first phase of the multiphase SiAlON material is alpha SiAION having the general formula

        M.sub.x Si.sub.12-m-n Al.sub.m+n O.sub.n N.sub.16-n

wherein o<x≤2 and M is a multi-cationic mixture including

(i) element of calcium;

(ii) at least one of yttrium and/or a rare earth element with atomic number greater than 62;

(iii) at least one of a rare earth element with atomic number equal or smaller than 62.

3. The ceramic material of claim 1, wherein the second phase of the multiphase SiAlON material is beta SiAlON having the general formula

        Si.sub.6-z Al.sub.z O.sub.z Nsub.8-z

wherein 0<z<4.2; the value of z is 0<z<1.6

4. The ceramic material of claim 1, wherein the third phase of the multiphase SiAlON material is an intergranular amorphous and/or crystalline nature containing in addition to elements of Si, Al, O and N,

(i) element of calcium;

(ii) at least one of yttrium and/or a rare earth element with atomic number greater than 62;

(iii) at least one of rare earth element with atomic number equal or smaller than 62.

5. The ceramic material of claim 1 and other claims, wherein the intergranular phase is present in an amount between 0-16 percent by volume based upon total material volume.

6. The ceramic material of claim 1 and other claims, wherein the weight ratio of alpha SiAlON to beta SiAlON is changing from about 20:80 to about 80:20

7. A method of producing a SiAlON ceramic material having at least a first, second and third phase comprising the steps of preparing a mixture of Si.sub.3 N.sub.4, AlN, Al.sub.2 O.sub.3 and a multi-cation mixture composition which includes a compound selected from the group consisting of oxides and nitrides of calcium; at least one compound selected from the group consisting of oxides and nitrides of yttrium and/or a rare earth element with atomic number greater than 62; and at least one compound selected from the group consisting of oxides and nitrides of a rare earth element with atomic number equal or smaller than 62; attrition milling them in water; drying the mixture; pressing at 150 MPa; sintering between 1600-1850.degree.C at least 18 minutes in a gas pressure sintering furnace at a pressure between 1-100 bar.

8. The method of claim 7, wherein a mixture of Si.sub.3 N.sub.4, A1N, Al.sub.2 O.sub.3 and a multi-cation mixture composition which includes a compound selected from the group consisting of oxides and nitrides of calcium; at least one compound selected from the group consisting of oxides and nitrides of yttrium and/or a rare earth element with atomic number greater than 62; and at least one compound selected from the group consisting of oxides and nitrides of a rare earth element with atomic number equal or smaller than 62 is attrition milled in water.

9. The method of claim 7, wherein the attrition milled mixture is dried; pressed at 150 MPa; sintered between 1600-1850.degree.C for at least 18 minutes in a gas pressure sintering furnace at a pressure between 1-100 bar.

Ansprüche

1. Mehrphasiger SiAlON-Keramikwerkstoff mit mindestens einer ersten, einer zweiten und einer dritten Phase, umfassend

a) eine erste Phase von α-SiAlON der allgemeinen Formel M_xSi_12-m-nAl_m+nO_nN_16-n, wobei 0 < x ≤ 2 ist und M ein Gemisch mehrerer Kationen, umfassend

(i) das Element Calcium;

(ii) mindestens ein Element aus Yttrium und/oder einem Seltenerd-Element mit einer Atomzahl größer 62;

(iii) mindestens ein Seltenerd-Element mit einer Atomzahl gleich oder kleiner 62;

b) eine zweite Phase von β-SiAlON der allgemeinen Formel Si_6-zAl_zO_zN_8-z, wobei 0 < z ≤ 4,2 ist und der z-Wert 0 < z < 1,6 ist;

c) eine dritte Phase, die intergranulär amorph und/oder kristallin ist, die neben den Elementen Si, Al, O und N,

(i) das Element Calcium enthält;

(ii) mindestens ein Element aus Yttrium und/oder einem Seltenerd-Element mit einer Atomzahl größer 62;

(iii) mindestens ein Seltenerd-Element mit einer Atomzahl gleich oder kleiner 62.

2. Keramikwerkstoff nach Anspruch 1, wobei die erste Phase des mehrphasigen SiAlON-Materials α-SiAlON ist der allgemeinen Formel M_xSi_12-m-nAl_m+nO_nN_16-n, wobei 0 < x ≤ 2 ist und M ein Gemisch mehrerer Kationen, umfassend

(i) das Element.Calcium;

(ii) mindestens ein Element aus Yttrium und/oder einem Seltenerd-Element mit einer Atomzahl größer 62;

(iii) mindestens ein Seltenerd-Element mit einer Atomzahl gleich oder kleiner 62.

3. Keramikwerkstoff nach Anspruch 1, wobei die zweite Phase des mehrphasigen SiAlON-Material β-SiAlON der allgemeinen Formel Si_6-zAlO_zN_8-z ist, wobei 0 < z ≤ 4,2 ist und der z-Wert 0 < z < 1,6.

4. Keramikwerkstoff nach Anspruch 1, wobei die dritte Phase des mehrphasigen SiAlON-Materials von intergranulärer amorpher und/oder kristalliner Natur ist und neben den Elementen Si, Al, O und N umfasst,

(i) das Element Calcium;

(ii) mindestens ein Element von Yttrium und/oder einem.Seltenerd-Element mit einer Atomzahl größer 62;

(iii) mindestens ein Element von einem Seltenerd-Element mit einer Atomzahl gleich oder kleiner 62.

5. Keramikwerkstoff nach Anspruch 1 und weiteren Ansprüchen, wobei die intragranuläre Phase in einer Menge zwischen 0 bis 16 Vol.%, bezogen auf das Gesamtvolumen des Materials, zugegen ist.

6. Keramikwerkstoff nach Anspruch 1 und weiteren Ansprüchen, wobei das Gewichtsverhältnis von α-SiAlON zu β-SiAlON von etwa 20:80 bis etwa 80:20 reicht.

7. Verfahren zur Herstellung eines SiAlON-Keramikwerkstoffs mit mindestens einer ersten, einer zweiten und einer dritten Phase, umfassend die Schritte:

Herstellen eines Gemisches von Si₃N₄, AlN, Al₂O₃ sowie einer Zusammensetzung mit einem Gemisch mehrerer Kationen, das mindestens eine Verbindung umfasst aus der Gruppe der Oxide und Nitride des Calciums, mindestens einer Verbindung aus der Gruppe der Oxide und Nitride des Yttriums und/oder eines Seltenerd-Elements mit einer Atomzahl größer 62; mindestens eine Verbindung aus der Gruppe der Oxide und Nitride eines Seltenerd-Elements mit einer Atomzahl gleich oder kleiner 62;

Zerreibmahlen dieser in Wasser;

Trocknen des Gemisches;

Pressen bei 150 MPa;

mindestens 18-minütiges Sintern zwischen 1600 und 1850°C in einem Gasdruck-Sinterofen unter einem Druck zwischen 1 und 100 bar.

8. Verfahren nach Anspruch 7, wobei in Wasser zerreibgemahlen werden ein Gemisch von Si₃N₄, AlN, Al₂O₃ und einer Zusammensetzung mit einem Gemisch mehrerer Kationen, umfassend eine Verbindung aus der Gruppe der Oxide und Nitride des Calciums, mindestens einer Verbindung aus der Gruppe der Oxide und Nitride des Yttriums und/oder eines Seltenerd-Elements mit einer Atomzahl größer 62; und mindestens eine Verbindung aus der Gruppe der Oxide und Nitride eines Seltenerd-Elements mit einer Atomzahl gleich oder kleiner 62.

9. Verfahren nach Anspruch 7, wobei das zerreibgemahlene Gemisch getrocknet wird, unter 150 MPa gepresst wird, mindestens 18 Minuten zwischen 1600 bis 1850°C in einem Gasdruck-Sinterofen unter einem Druck zwischen 1 bis 100 bar gesintert wird.

Revendications

1. Matériau céramique de type SiAlON à phases multiples, comportant au moins une première phase, une deuxième phase et une troisième phase, à savoir :

a) une première phase de SiAlON alpha, de formule générale

        M_xSi_12-m-nAl_m+nO_nN_16-n

dans laquelle l'indice x est supérieur à 0 et inférieur ou égal à 2,
et M représente un mélange de plusieurs cations comprenant

i) de l'élément calcium,

ii) au moins l'un de l'élément yttrium et/ou des éléments terres rares dont le numéro atomique est supérieur à 62,

iii) et au moins l'un des éléments terres rares dont le numéro atomique est inférieur ou égal à 62 ;

b) une deuxième phase de SiAlON bêta, de formule générale

        Si_6-zAl_zO_zN_8-z

dans laquelle l'indice z est un nombre supérieur à 0 et inférieur à 4,2, dont la valeur est supérieure à 0 et inférieure à 1,6,

c) et une troisième phase intergranulaire, de nature amorphe et/ou cristalline, contenant, outre des éléments silicium, aluminium, oxygène et azote :

i) de l'élément calcium,

ii) au moins l'un de l'élément yttrium et/ou des éléments terres rares dont le numéro atomique est supérieur à 62,

iii) et au moins l'un des éléments terres rares dont le numéro atomique est inférieur ou égal à 62.

2. Matériau céramique conforme à la revendication 1, dans lequel la première phase du matériau de type SiAlON à phases multiples est une phase de SiAlON alpha de formule générale

        M_xSi_12-m-nAl_m+nO_nN_16-n

dans laquelle l'indice x est supérieur à 0 et inférieur ou égal à 2, et M représente un mélange de plusieurs cations comprenant

i) de l'élément calcium,

ii) au moins l'un de l'élément yttrium et/ou des éléments terres rares dont le numéro atomique est supérieur à 62,

iii) et au moins l'un des éléments terres rares dont le numéro atomique est inférieur ou égal à 62.

3. Matériau céramique conforme à la revendication 1, dans lequel la deuxième phase du matériau de type SiAION à phases multiples est une phase de SiAION bêta de formule générale

        Si_6-zAl_zO_zN_8-z

dans laquelle l'indice z est un nombre supérieur à 0 et inférieur à 4,2, dont la valeur est supérieure à 0 et inférieure à 1,6.

4. Matériau céramique conforme à la revendication 1, dans lequel la troisième phase du matériau de type SiAION à phases multiples est une phase intergranulaire, de nature amorphe et/ou cristalline, contenant, outre des éléments silicium, aluminium, oxygène et azote :

i) de l'élément calcium,

ii) au moins l'un de l'élément yttrium et/ou des éléments terres rares dont le numéro atomique est supérieur à 62,

iii) et au moins l'un des éléments terres rares dont le numéro atomique est inférieur ou égal à 62.

5. Matériau céramique conforme à la revendication 1 et autres revendications, dans lequel la phase intergranulaire se trouve présente en une quantité représentant 0 à 16 % du volume total du matériau.

6. Matériau céramique conforme à la revendication 1 et autres revendications, dans lequel le rapport pondéral de la phase de SiAlON alpha à la phase de SiAION bêta vaut d'environ 20/80 à environ 80/20.

7. Procédé de production d'un matériau céramique de type SiAlON comportant au moins une première phase, une deuxième phase et une troisième phase, lequel procédé comporte les étapes suivantes :

- préparer un mélange de nitrure de silicium Si₃N₄, de nitrure d'aluminium AIN, d'alumine Al₂O₃, et d'une composition comprenant un mélange de plusieurs cations, qui contient un composé choisi parmi l'oxyde de calcium et le nitrure de calcium, au moins un composé choisi dans l'ensemble formé par les oxydes et nitrures d'yttrium et des éléments terres rares dont le numéro atomique est supérieur à 62, et au moins un composé choisi dans l'ensemble formé par les oxydes et nitrures des éléments terres rares dont le numéro atomique est inférieur à 62;

- moudre le tout, par attrition dans de l'eau ;

- faire sécher le mélange ;

- le comprimer sous une pression de 150 MPa;

- et fritter le mélange comprimé en le portant à une température de 1600 à 1850 °C durant au moins 18 minutes, dans un four de frittage à gaz sous pression, sous une pression de 1 à 100 bars.

8. Procédé conforme à la revendication 7, dans lequel on moud, par attrition dans de l'eau, un mélange de nitrure de silicium Si₃N₄, de nitrure d'aluminium AIN, d'alumine Al₂O₃, et d'une composition comprenant un mélange de plusieurs cations, qui contient un composé choisi parmi l'oxyde de calcium et le nitrure de calcium, au moins un composé choisi dans l'ensemble formé par les oxydes et nitrures d'yttrium et des éléments terres rares dont le numéro atomique est supérieur à 62, et au moins un composé choisi dans l'ensemble formé par les oxydes et nitrures des éléments terres rares dont le numéro atomique est inférieur à 62.

9. Procédé conforme à la revendication 7, dans lequel on fait sécher le mélange moulu par attrition, puis on le comprime sous une pression de 150 MPa, et l'on fritte le mélange comprimé en le portant à une température de 1600 à 1850 °C durant au moins 18 minutes, dans un four de frittage à gaz sous pression, sous une pression de 1 à 100 bars.